Demodulators, particularly I/Q demodulators, are important switching elements for mobile radio. They generally convert a received radio-frequency signal to an intermediate frequency and output it as a complex signal at the output. In this case, the complex signal has been broken down into an in-phase component I and a quadrature-phase component Q. The two components I and Q are filtered from higher-order signal components (which are produced during conversion) by means of a low-pass filter and are supplied to further signal processing.
FIG. 5 shows an I/Q demodulator of this type having two double-balanced mixers. The two balanced mixers are also referred to as Gilbert mixers. The I/Q demodulator contains two Gilbert mixers, one of which is used to convert the in-phase component I and the other of which is used to convert the quadrature-phase component Q. The demodulator shown here is designed to process differential signals. The radio-frequency signal (in the form of a differential signal RF+ and RF− having two components) is supplied, via the capacitors C2, to the control connections of the transistors T2 in the two Gilbert mixers. The two capacitors C2 are used to decouple the RF path from the bias voltage Vb which is used to set the operating point of the transistors T2. The source/emitter circuits (operating as voltage/current converters) in the transistors T2 have been combined to form a differential amplifier with negative current feedback and are driven in opposite senses by the RF signals.
The output signals from these transistors are each supplied to a Gilbert cell comprising two differential amplifiers which are operated as changeover switches. The local oscillator signal LOI or LOQ is likewise applied in the form of a differential signal to the control connections of the transistors T1 which form the Gilbert cell. The current signal outputs of the two Gilbert mixers in the demodulator 1 are each connected to the potential VDD via a resistor R1. As a result, the resistor is used to convert the current signal, at the current interface of the demodulator, to an output voltage which can be tapped off in the form of a differential voltage signal at the taps OUTI− and OUTI+ and OUTQ− and OUTQ+.
For reasons of linearity, it is usually necessary to implement a passive low-pass filter at the output of the two Gilbert mixers. This filter is simultaneously part of the channel filter. As can be seen in the embodiment in FIG. 5, a capacitor C1 is therefore connected between the two signal paths of each mixer. The capacitor C1, together with the two resistors R1, produces a low-pass filter.
In some narrowband systems, for example in the GSM standard, the first pole point of the low-pass filter must be in the region of some few hundred kHz. In this case, the cut-off frequency results from the resistance and the capacitance.
However, the size of the resistor R1 is limited by the output voltage signal. An excessively large resistance value for the resistor R1, combined with the direct current flowing through the resistor, results in an excessively small voltage at the output OUTI−, OUTI+ and OUTQ−, OUTQ+. Therefore, the resistance value must not be selected to be too large. In addition, since the output resistance of the mixer is also limited by the supply voltage VDD, very large capacitance values of C1 are needed to implement the low-pass filter. However, large capacitance values can be integrated only by using a considerable amount of surface area and thus increase the costs of the integrated circuit.